1. Field of the Invention
The present invention relates to a structure for a feed belt driving sprocket in a paper feeder, which can be so fitted on a drive shaft that it is absolutely immovable in the direction of rotation.
2. Description of the Prior Art
The drive of the feed belt of the paper feeder is accomplished by means of a sprocket.
Specifically, a feed belt is fitted on both a sprocket, which is borne at one-end portions of a pair of side frames opposed to each other at a predetermined spacing, and a mounting member disposed at the other end portions of the same, and is circulated to run by the driving rotations of the sprocket.
The sprocket now in use at present and the structure for bearing the sprocket in the paired side frames are shown in FIGS. 17 to 19.
A sprocket 51 has its body 52 protruding at its two sides to form fitted portions 53. The sprocket 51 is rotatably borne in a pair of side frames 54a and 54b by having its fitted portions 53 fitted in fitting holes 55 formed in the side frames 54a and 54b. Moreover, the sprocket 51 is formed with a square-shaped axial hole 56 so that it is rotated by inserting a square-shaped drive shaft 57 into the axial hole 56 and by rotating the drive shaft 57.
As quite natural, the spacing (A') between the opposed sides of the drive shaft 57 is made smaller than the spacing (B') between the opposed inner peripheral sides of the axial hole 56 so that a slight clearance is left between the axial hole 56 and the drive shaft 57 fitted in the former. As a result, as shown in FIG. 19, the sprocket 51 is slightly moved in its rotating direction from the drive shaft 57 so that a phase difference is established for each rotation during the paper feed between the drive shaft 57 and the sprocket 51 to deteriorate the printing accuracy of the printer.
In order to prevent this drawback, it is advisable to minimize the clearance between the axial hole 56 of the sprocket 51 and the drive shaft 57 as much as possible. However, a pair of paper feeders have to be used to adjust their gap in accordance with the paper width.
For this necessity, the paper feeders have to be moved as a whole along the drive shafts 57 by a weak force. For an excessively small clearance, therefore, the drive shaft 57 can be inserted into the axial hole 56 of the sprocket 51, but the whole paper feeder can hardly be moved along the drive shaft 57, or the drive shaft 57 may be unable to be inserted into the axial hole 56 of the sprocket 51, as the case may be.
As a result, the sprocket of the single structure has found it difficult to simultaneously achieve the elimination of the phase difference of each rotation between the drive shaft and the sprocket and the movement of the whole paper feeder by the small force. This simultaneous achievement of the two requirements could be effected if the sprocket were equipped with a special part for preventing the sprocket from moving in the direction of rotation relative to the drive shaft. In this case, however, the equipment will raise the production cost and cannot expect a high accuracy.